I bought and starting using the O3 at the end of November primarily for its sleep phase metrics.
Regarding HRV, I have the same results as you. As you know, Oura’s HR is wrong (for me doesn’t go above 130 even during SS/threshold), so my assumption is that if it can’t accurately capture HR, how can it accurately capture HRV, at least as of yet?
We’ll see if it (HR and HRV) improves once Workout HR feature improved and particularly after they introduce their new sleep algorithm that will begin to use its new sensors. However, in all cases chest HRM straps are going to give you more accurately results than finger or wrist (i.e. Whoop) estimators.